The present invention relates to a method for the preparation of silicon carbide whiskers or, more particularly, to an inexpensive and easy method for the preparation of silicon carbide whiskers from industrially useless silicon compounds available as the by-products of various processes as the starting material in a high yield.
As is known, silicon carbide whiskers have outstandingly high mechanical strength, elastic modulus, oxidation resistance, heat resistance and chemical stability in comparison with other fibrous materials used hitherto for the reinforcement of composite materials and also have good wettability on various kinds of metals. Therefore, silicon carbide whiskers are recently high-lighted as the reinforcing fibers in composite materials with various kinds of ceramics, metals, plastics and others as the matrix.
Several methods are known for the preparation of silicon carbide whiskers including (1) a method of solid-phase reduction of silicon dioxide with carbon, (2) a method of high-temperature gas-phase reaction of an organosilicon compound or a mixture of a silicon compound and a carbon compound and (3) a method of vapor-phase whisker growing by the reaction of a fluorine-containing silicate and carbon. These methods have their respective problems and disadvantages. Although the second and the third methods mentioned above can be relatively easily performed for the production of a small quantity of the whiskers in a laboratory-scale, for example, great difficulties are unavoidable when a process for the mass production of the whiskers should be established in large facilities of industrial scale because the reaction must be performed at an extremely high temperature in addition to the inflammability of the reactant gases and corrosiveness of the by-product gases. In contrast thereto, the first method is advantageous because no inflammable gases are used in the process to facilitate construction of large facilities for mass production and a three-dimensionally isotropic whisker preform can be easily manufactured since the whiskers produced by the method are in a powdery form. Accordingly, several modifications of this method of solid-phase reduction have been developed including (a) a method in which silicon carbide whiskers are deposited in the course of the production of silicon carbide blocks by the Acheson's method, (b) a method utilizing the siliceous constituents contained in seed hulls of gramineous plants as the starting material, (c) a method using silica sand as the starting material, (d) a method using glass wastes as the starting material and (e) a method in which the starting material is a synthetically prepared silica material having a large specific surface area such as silica sols, silica gels and the like.
These prior art methods are also not free from several problems and disadvantages of their own. For example, the method (a) is inherently not suitable for industrialization because deposition of the silicon carbide whiskers depends merely on eventuality. The method (b) involves disadvantages that the seed hulls as the starting material must be subjected to a pretreatment of carbonization or ashing prior to the reaction while a large equipment is required for the pretreatment because the content of silica in the seed hulls rarely exceeds 20% by weight. The method (c) also requires a pretreatment of the silica sand into a finely pulverized powder while even an extremely finely pulverized powder of silica sand, which is a crystalline material, has low reactivity due to the crystallinity thereof in comparison with amorphous siliceous materials so that the productivity of the silicon carbide whiskers cannot be high enough. The method (d) is practically not feasible because the starting material usually contains a large amount of metallic impurities other than silicon which are more or less unavoidably taken into the silicon carbide whiskers as the product to cause degradation of the product quality. Moreover, some of the metallic impurities are volatile at high temperatures so that, when the starting material is heated, such impurities are dissipated in the form of vapor which is condensed and deposited on the surface of a cold part of the apparatus to inhibit continuous running of the process. The method (e) is economically not advantageous due to the expensiveness of the starting siliceous material as a silicon source. Moreover, the initially large specific surface area of the starting material is gradually decreased as the temperature of the material is increased to cause shrinkage and the silica material loses the reactivity of the surface along with decrease in the specific surface area. Thus, none of the above described prior art methods is quite satisfactory from the standpoint of industrial production of silicon carbide whiskers.